The patch titled epoll: fix spurious lockdep warnings has been added to the -mm tree. Its filename is epoll-fix-spurious-lockdep-warnings.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find out what to do about this The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: epoll: fix spurious lockdep warnings From: Nelson Elhage <nelhage@xxxxxxxxxxx> epoll can acquire recursively acquire ep->mtx on multiple "struct eventpoll"s at once in the case where one epoll fd is monitoring another epoll fd. This is perfectly OK, since we're careful about the lock ordering, but it causes spurious lockdep warnings. Annotate the recursion using mutex_lock_nested, and add a comment explaining the nesting rules for good measure. Recent versions of systemd are triggering this, and it can also be demonstrated with the following trivial test program: --------------------8<-------------------- int main(void) { int e1, e2; struct epoll_event evt = { .events = EPOLLIN }; e1 = epoll_create1(0); e2 = epoll_create1(0); epoll_ctl(e1, EPOLL_CTL_ADD, e2, &evt); return 0; } --------------------8<-------------------- Reported-by: Paul Bolle <pebolle@xxxxxxxxxx> Tested-by: Paul Bolle <pebolle@xxxxxxxxxx> Signed-off-by: Nelson Elhage <nelhage@xxxxxxxxxxx> Cc: Dave Jones <davej@xxxxxxxxxx> Cc: Davide Libenzi <davidel@xxxxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/eventpoll.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-) diff -puN fs/eventpoll.c~epoll-fix-spurious-lockdep-warnings fs/eventpoll.c --- a/fs/eventpoll.c~epoll-fix-spurious-lockdep-warnings +++ a/fs/eventpoll.c @@ -70,6 +70,15 @@ * simultaneous inserts (A into B and B into A) from racing and * constructing a cycle without either insert observing that it is * going to. + * It is necessary to acquire multiple "ep->mtx"es at once in the + * case when one epoll fd is added to another. In this case, we + * always acquire the locks in the order of nesting (i.e. after + * epoll_ctl(e1, EPOLL_CTL_ADD, e2), e1->mtx will always be acquired + * before e2->mtx). Since we disallow cycles of epoll file + * descriptors, this ensures that the mutexes are well-ordered. In + * order to communicate this nesting to lockdep, when walking a tree + * of epoll file descriptors, we use the current recursion depth as + * the lockdep subkey. * It is possible to drop the "ep->mtx" and to use the global * mutex "epmutex" (together with "ep->lock") to have it working, * but having "ep->mtx" will make the interface more scalable. @@ -464,13 +473,15 @@ static void ep_unregister_pollwait(struc * @ep: Pointer to the epoll private data structure. * @sproc: Pointer to the scan callback. * @priv: Private opaque data passed to the @sproc callback. + * @depth: The current depth of recursive f_op->poll calls. * * Returns: The same integer error code returned by the @sproc callback. */ static int ep_scan_ready_list(struct eventpoll *ep, int (*sproc)(struct eventpoll *, struct list_head *, void *), - void *priv) + void *priv, + int depth) { int error, pwake = 0; unsigned long flags; @@ -481,7 +492,7 @@ static int ep_scan_ready_list(struct eve * We need to lock this because we could be hit by * eventpoll_release_file() and epoll_ctl(). */ - mutex_lock(&ep->mtx); + mutex_lock_nested(&ep->mtx, depth); /* * Steal the ready list, and re-init the original one to the @@ -670,7 +681,7 @@ static int ep_read_events_proc(struct ev static int ep_poll_readyevents_proc(void *priv, void *cookie, int call_nests) { - return ep_scan_ready_list(priv, ep_read_events_proc, NULL); + return ep_scan_ready_list(priv, ep_read_events_proc, NULL, call_nests + 1); } static unsigned int ep_eventpoll_poll(struct file *file, poll_table *wait) @@ -737,7 +748,7 @@ void eventpoll_release_file(struct file ep = epi->ep; list_del_init(&epi->fllink); - mutex_lock(&ep->mtx); + mutex_lock_nested(&ep->mtx, 0); ep_remove(ep, epi); mutex_unlock(&ep->mtx); } @@ -1134,7 +1145,7 @@ static int ep_send_events(struct eventpo esed.maxevents = maxevents; esed.events = events; - return ep_scan_ready_list(ep, ep_send_events_proc, &esed); + return ep_scan_ready_list(ep, ep_send_events_proc, &esed, 0); } static inline struct timespec ep_set_mstimeout(long ms) @@ -1267,7 +1278,7 @@ static int ep_loop_check_proc(void *priv struct rb_node *rbp; struct epitem *epi; - mutex_lock(&ep->mtx); + mutex_lock_nested(&ep->mtx, call_nests + 1); for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) { epi = rb_entry(rbp, struct epitem, rbn); if (unlikely(is_file_epoll(epi->ffd.file))) { @@ -1409,7 +1420,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, in } - mutex_lock(&ep->mtx); + mutex_lock_nested(&ep->mtx, 0); /* * Try to lookup the file inside our RB tree, Since we grabbed "mtx" _ Patches currently in -mm which might be from nelhage@xxxxxxxxxxx are epoll-fix-spurious-lockdep-warnings.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html